1. Field of the Invention
The present invention relates to a device having a scanning type carrier suitable for scanning a printing head, read out sensor and other devices or elements as a functional element, and a printing apparatus.
2. Description of the Related Art
Conventionally, in an ink-jet printing apparatus performing printing by scanning an ink-jet printing head, there are some printing apparatus, each of which is provided with a transmission type optical sensor on a carriage mounting the ink-jet printing head and moving in a primary scanning direction, and a blocking plate for blocking an optical path of the optical sensor on a predetermined position of a main body of the apparatus. The optical sensor outputs a detection signal when the optical path thereof is blocked by the blocking plate. The printing apparatus is responsive to the detection signal to control a moving position of the carriage for fixing a printing start position in printing characters or the like, and for other operations. Conversely, there are alternative examples, in which the blocking plate is provided on the carriage and the optical sensor is provided on the main body of the apparatus.
On the other hand, the inventor has already proposed an ink-jet printing apparatus, in which two carriages mounting two different printing heads are selectively coupled to one scanning body as a carrier (Japanese Patent Application Laying-open No. 164432/1994). In such an apparatus, two carriages are selectively coupled to the carrier depending upon kind of document, such as a document primarily consisting of characters, a document, in which a character and graphic image are mixed, a document primarily consisting of graphic images and other documents, and perform high speed printing using the printing head on the carriage coupled to the carrier. In addition to this construction, a capping means for capping ink ejection openings of the printing head, a pump means with a tube extending within a range of motion of the scanning body for externally discharging an ink within the capping means, and a tube depressing means provided on the scanning body for squeezing the tube, may be provided. In this case, it becomes possible to perform a so-called recovery operation for maintaining the printing head in good condition, such as an operation for forcedly ejecting the ink through the ink ejection openings.
However, when the conventional carriage moving position control means is applied for the printing apparatus, in which the scanning body as the carrier and the carriages are provided separately, the following drawbacks are encountered.
At first, when the optical sensor is provided on one of the scanning body as the carrier or the main body of the printing apparatus, and the blocking plate is provided on the other, for controlling moving position of the carriage on the basis of the detection signal output upon blocking of the optical path of the optical sensor, the following problem can be caused. Namely, due to a certain amount of play between the scanning body and the carriage and a fluctuation in dimension of the scanning body, a printing position with respect to a paper as a printing medium can fluctuate significantly for each individual printing apparatus.
On the other hand, when bidirectional printing to perform printing in both a forward scanning movement and reverse scanning movement of the carriage, the following problem can arise. Namely, due to a difference of play amount between the scanning body and the carriage in the forward scanning motion and the reverse scanning motion, or a difference of driving condition, such as belt tension of a carrier driving belt in the forward scanning motion and in the reverse scanning motion, there may be a danger that the printing position in the forward scanning motion deviates from that in the reverse scanning motion. Such deviation of the printing position should result in degradation of image quality, such as fluctuation of character writing position or misalignment of rule lines. As a method for eliminating such fluctuation of the printing position, there is a method to shift ejection timing of the ink during the reverse scanning movement for a period corresponding to fluctuation of the printing position, in a case of bidirectional printing operation, for preventing the image quality from being degraded. In order to automatically calculate the period (bidirectional correction amount) corresponding to the fluctuation of the printing position, the detection signal of the optical sensor provided on one of the scanning body as the carrier or the main body of the apparatus may be used. For instance, with taking a timing of blocking of the optical path of the optical sensor as reference, a predetermined number of driving pulses are input to a pulse motor for scanning, and a time difference between the forward movement and the reverse movement when the scanning body is driven reciprocally in a magnitude corresponding to the predetermined number of driving pulses, is derived. The time difference thus derived is set as the bidirectional correction amount. However, in this case, the correction amount does not contain the error component between the scanning body and the carriage. Therefore, accurate bidirectional correction is difficult to satisfactorily prevent degradation of quality of the printed image. Such a drawback may be resolved by providing the optical sensor on one of the carriage and the main body of the apparatus, and the blocking plate on the other. However, in such a case, another new problem can arise in that when the scanning body is solely moved such as upon the foregoing recovery operation, an initial setting of the moving position and accurate control of the moving position of -the scanning body, i.e. carrier, becomes impossible.